Keck Observatory on Maunakea spots biggest pair of black hole jets ever seen

Astronomers on Maunakea on Hawai‘i Island have spotted the biggest pair of black hole jets ever seen, spanning 23 million light-years in total length – equivalent to 140 Milky Way
galaxies lined up back-to-back.

“This pair is not just the size of a solar system, or a Milky Way; we are talking about 140 Milky
Way diameters in total,” said Caltech postdoctoral scholar Martijn Oei.

Oei is the lead author of a new study, which includes data from W. M. Keck Observatory on Maunakea, published in the journal Nature.

The jet megastructure, nicknamed Porphyrion after a giant in Greek mythology, dates to a time when our universe was 6.3 billion years old, or less than half its present age of 13.8 billion years. These outflows – with a total power output equivalent to trillions of suns – shoot out from above and below a supermassive black hole at the heart of a remote galaxy.

Prior to Porphyrion’s discovery, the largest confirmed jet system was Alcyoneus, also named
after a giant in Greek mythology. Alcyoneus, which was discovered in 2022 by the same team
that found Porphyrion, spans the equivalent of around 100 Milky Ways. The latest finding suggests that these giant jet systems may have had a larger influence on the formation of galaxies in the young universe than previously believed.

Porphyrion existed during an early epoch when the wispy filaments that connect and feed galaxies, known as the cosmic web, were closer together than they are now. That means enormous jets like Porphyrion reached across a greater portion of the cosmic web compared to jets in the local universe.

“Astronomers believe that galaxies and their central black holes co-evolve, and one key aspect
of this is that jets can spread huge amounts of energy that affect the growth of their host
galaxies and other galaxies near them,” said co-author George Djorgovski, professor of
astronomy and data science at Caltech.

To find the galaxy from which Porphyrion originated, the team used the Giant Metrewave Radio
Telescope in India along with ancillary data from a project called Dark Energy Spectroscopic
Instrument, which operates from Kitt Peak National Observatory in Arizona. The observations
pinpointed the home of the jets to a hefty galaxy about 10 times more massive than our Milky
Way.

The team then used the Keck Observatory to show that Porphyrion is 7.5 billion light-years from Earth.

“Up until now, these giant jet systems appeared to be a phenomenon of the recent universe,”
Oei said. “If distant jets like these can reach the scale of the cosmic web, then every place in
the universe may have been affected by black hole activity at some point in cosmic time.”

Keck Observatory’s Low Resolution Imaging Spectrometer also revealed that Porphyrion emerged from what is called a radiative-mode active black hole, as opposed to one that is in a
jet-mode state. When supermassive black holes become active – in other words, when their
immense forces of gravity tug on and heat up surrounding material – they are thought to either
emit energy in the form of radiation or jets.

Radiative-mode black holes were more common in the young, or distant, universe, while jet-mode ones are more common in the present-day universe. The fact that Porphyrion came from a radiative-mode black hole came as a surprise because astronomers did not know this mode could produce such huge and powerful jets.

Because Porphyrion lies in the distant universe where radiative-mode black holes abound, the finding implies there may be a lot more colossal jets left to be found.

As a next step, Oei wants to better understand how these megastructures influence their surroundings. The jets spread cosmic rays, heat, heavy atoms and magnetic fields throughout
the space between galaxies. Oei is specifically interested in finding out the extent to which giant jets spread magnetism.

“The magnetism on our planet allows life to thrive, so we want to understand how it came to be,” he said. “We know magnetism pervades the cosmic web, then makes its way into galaxies and stars, and eventually to planets, but the question is: Where does it start? Have these giant jets spread magnetism through the cosmos?”